This invention relates to power amplification required on telephone transmission facilities, and more particularly to a novel amplifier by which to vary the power level of telephone line signals quickly and reliably in discrete steps.
Telephone line amplifiers are needed to compensate for signal loss occurring as a function of distance along the transmission facility, often a metallic cable pair. The amplifiers must be spaced closely enough along the facility to prevent the signal from dropping near to the noise level existing on the line, for otherwise the signal cannot be recovered effectively without resorting to sophisticated and costly techniques. For bi-directional two-wire applications, negative impedance or hybrid type repeaters are used. For uni-directional two-wire applications, simpler uni-directional line amplifiers can be used. This type of uni-directional amplifier is referred to hereinafter as "telephone line amplifier."
Telephone line amplifiers provided heretofore incorporate a variety of techniques to control gain or amplification factor, all of which are characterized by presenting some difficulty in determining quickly and accurately the required amplifier gain, whether during installation or during routine testing.
One such prior amplifier utilizes "gain marked" screw-down adjustments. Another utilizes a selection of wire jumpers to select the gain. Still another utilizes a variable potentiometer adjustment, requiring the use of meters to set the desired gain.
The foregoing prior techniques normally require that the amplifier be removed from the normal operational position in the equipment shelf during adjustment. In all cases the adjustment requires considerable time, thereby reflecting adversely on the overall cost of installation and maintenance associated with these prior techniques.